Electronic assembly video inspection system

ABSTRACT

An apparatus for automatically assessing the quality of a printed circuit board assembly using digitized video image analysis. The apparatus integrates with existing relatively low precision automated surface mount technology (“SMT”) manufacturing systems as an inspection station insertable at various steps in the assembly process or as a separate manually loaded station. The inspection station includes a high resolution video imaging system and a video image analyzer comprising an onboard master computer that generates control signals to reposition the camera mounted within a screen on a movable carriage and/or reposition the circuit board, and adjust the lighting; and generates individual board status data to be archived, graphically displayed on monitors or otherwise utilized by a rework station.

PRIOR APPLICATION

This is a divisional application of Ser. No. 10/682,262 now abandonedfiled Oct. 8, 2003 and a divisional application of Ser. No. 09/486,234filed Feb. 23, 2000 now U.S. Pat. No. 6,681,038, a 371 of PCT/US98/21383filed Oct. 8, 1998, and a continuation of Ser. No. 08/947,756 nowabandoned filed Oct. 9, 1997; and provisional application Ser. No.60/028,451 filed Oct. 9, 1996.

FIELD OF THE INVENTION

This invention relates to automated assembly mechanisms for electroniccomponents, quality assurance, and more particularly to devices used inassessing whether components have been assembled adequately.

BACKGROUND OF THE INVENTION

The ever-increasing miniaturization of electronic components modules andassemblies and the market pressures for cost reduction has made theassembly of those devices a precise, automated, multi-step task. Mostdevices are assembled using surface mount technology (“SMT”) whereinscores, if not hundreds of individual components are precisely placedand soldered on at least one printed circuit board in an “assembly line”fashion.

Printed circuit boards travel successively, in-line along conveyorsthrough a series of stations which perform each step in the assemblyprocess. Typically, an empty board enters a solder paste delivery systemwhich places uncured solder paste on portions of the board requiringsoldered connections. The board then enters one or more chip shooterstations which physically place components on the board. The board thenproceeds through an oven which cures the solder paste. After cooling theboard is ready for testing and other finalization steps prior topackaging and shipment.

At each step there is a potential for errors to occur which result in adefective board. Some of the potential printed circuit board assemblydefects include: circuit board defects such as opens and shorts on thetraces; placement defects wherein components are missing, of the wrongtype, incorrectly oriented, or misaligned; solder defects in amount andplacement which can result in solder bridges on the leads ortomb-stoning of components caused by solder contraction during curing;and other defects such as damage caused by mechanical mishandling.

Previous procedures and devices for testing whether defects exist onfreshly assembled printed circuit boards suffer from various drawbacks.

Human testing and inspection is costly, slow and subject to a highdegree of inaccuracy. The devices used by human testers are typicallyheavy, bulky, and not readily portable. Electronic in-circuit testingsuffers from being slow and highly iterative in order to pinpoint thelocation of a defect and often cannot detect the most commonmanufacturing errors.

In order to minimize continued work on a board which has already becomedefective, manufacturers often provide for testing at several stagesduring assembly. However, a particular piece of automated test apparatusis usually designed to test a specific type of board, specific defects,and/or only at a specific point in the assembly. Therefore, numerousdifferent testing devices have been required.

Current automatic visual or other electromagnetic radiation basedinspection systems suffer from similar drawbacks. X-ray based systemsare suited to scan for metallic defects such as faulty traces andsubsurface defects. However, high resolution x-ray inspection isexpensive and time consuming, and potentially hazardous to nearby humanoperators.

In other systems, light produced by lamps or LEDs (“Light-EmittingDiodes”) is reflected off the surface being inspected into one or morevideo cameras. Some require the use of two images obtained underdifferent lighting conditions as disclosed in Takahashi, U.S. Pat. No.5,059,559. Other various digital and analog signal analyzing processescan be used to determine the existence of visually detectable defects.For example analyses have been made upon a monochrome intensitycomparison measurement of the signal corresponding to the image of thegaps between terminal leads.

These systems are relatively low resolution and hence slow. If thoroughinspection is required, the system must zoom in and successively scanportions of the board in a piecemeal fashion. In addition, monochromeintensity comparisons are prone to inaccuracies where adjacent featureshave similar intensities. An averagely populated, 3 inch by 5 inchboard, such as a standard PCI SVGA video adapter card will take about 1minute 20 seconds to inspect thoroughly.

Most prior systems require extremely precise location of the board andcamera, on the order of 0.001 of an inch. The board and camera must bemade resistant to vibration. The prior solution entailed a massiveplatform made of slate or other heavy materials, and precise,vibration-resistant board handling and camera carriage mechanisms. Mostprior systems weighed greater than 450 kilograms. These requirementsincrease the cost and lower the portability of the system.

Therefore, it is desirable to have an economical, automated testingsystem, which quickly detects the existence of the most prevalentmanufacturing defects, which determines automatically whether aparticular board may benefit from reworking and efficiently informs therework station of those defects; which keeps track of defects over timeto identify problems symptomatic to the assembly system; and which isquickly and easily moved to different points in the assembly line or outof the assembly line altogether for manual testing.

The instant invention results from an attempt to reduce cost, and toimprove the throughput and efficiency of automated assembly systems.

SUMMARY OF THE INVENTION

The objects of this invention are: to improve defect detection in theinspection and testing of electronic components assemblies; to moreefficiently direct repair; to allow for the monitoring over time of thequality of inspected boards, in order to improve the throughput ofautomatic assembly mechanisms; to allow portability of the inspectiondevice to various points on the assembly line and off the assembly linealtogether for manual testing.

These and other valuable objects are achieved by an automated,high-resolution, digital image inspection system, installed in aportable low-precision SMT type dispensing station. The systemidentifies the printed circuit board being inspected, and locatesvisually detectable defects thereon. The type and location of defectsare automatically associated with the board in a database, which isaccessible by a rework/repair station, and which allows for statisticalmonitoring of the history and status of an assembly's product.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a process flow block diagram of the populated board inspectionsystem as integrated in a surface mount technology assembly line;

FIG. 2 is a data flow block diagram of the inspection system accordingto the invention;

FIG. 3 is a diagrammatic perspective view of a first embodiment of theinspection station of the invention having a board handling conveyor;

FIG. 4 is a diagrammatic perspective view of an alternate embodiment ofthe inspection station of the invention having a drawer for manuallyloading boards;

FIG. 5 is a perspective view of the manual board positioning mechanismon the drawer;

FIG. 6 is a perspective view of a board securing oblong pier;

FIG. 7 is a cross-sectional view thereof with in-situ circuit boardtaken along line 7—7; and

FIG. 8 is a cross-sectional view thereof with in-situ circuit boardtaken along line 8—8.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawing, there is shown in FIG. 1 a functionalblock diagram of a typical SMT assembly line where printed circuitboards are assembled in a series of processing stations. Circuit boardsmove from solder paste application station 51, to one or more chipshooter stations 52, 53 and to an oven 54 for curing. An optionalfirst-in-first-out (“FIFO”) buffer station 55 may be used to connectstations which complete their tasks more sporadically, or in differentgroupings of boards. In the present embodiment the FIFO station 55 isused as a cooling station for groups of boards exiting the curing oven54 before entering the inspection station 56.

It should be noted that the inspection station 56 may be located betweenany one of the stations of the assembly process or as a stand alonemanually loaded station. However, the preferred location is immediatelyafter the boards have become fully populated, so as to prevent anyfurther processing of defective boards.

In its most automated embodiment, the inspection station 56 will befollowed by a board diverter 57 station which, according to the resultsprovided by the inspection station, will direct defective boards to arework station 58, and good boards to the next SMT processing station59. The rework station has access to results provided by the inspectionstation. Optionally, the rework station may employ an additionalinspection station 60 for inspecting reworked boards. Repaired boardswill re-enter the SMT processing at this point 61 as well.

The preferred inspection station is fully compatible with theinter-station communication protocol established by the Surface MountEquipment Manufacturer's Association (“SMEMA”), which allows for simplecommunication of board transport statuses to adjacent stations.

Referring now to FIG. 2, the preferred board inspection system isarranged in several functional units and subsystems. Automatic controlis handled by a master subsystem 71 as directed by a vision subsystem72. During automated “in-line” operation, the master subsystem generatescommands to the conveyor controller/interface 73 for activating boardhandling mechanisms, including a conveyor 5, and to the carriagecontroller/interface 75 for moving the carriage mounted camera 20, viadata communication lines 77, 78. A separate data line 79 carries zoomcommands to the camera 20. The master subsystem also carries out othertasks such as the archiving and flow of data to and from the visionsubsystem 72 and data entry and storage devices 82 and to the userinterface monitor 40. Another data line 91 communicates rework data toand from the rework station 58 and its control monitor. Data linespreferably conform to RS-232 or other well known digital datacommunications standards.

In this preferred embodiment of the invention, the camera 20 will outputa digital pulse train signal 93 comprising the encoded current videoimage to the vision subsystem 72. Alternatively, the camera 20 may issuea video signal which is converted to a digital signal by a digitizerintegrated or peripheral with the vision subsystem 72.

The vision subsystem 72 comprises a microprocessor controlled digitalsignal analyzer. Its primary function involves analyzing image datareceived via a data line 93 from the camera 20 in order to detect boarddefects. Part of this function involves signaling the master system 71to generate control commands for accepting and ejecting boards, drivingthe carriage in order to position the camera 20, and issuing camera zoomcommands. Configuration of the vision subsystem occurs through user dataentry and storage devices 82 directly or via the master subsystem 71.The vision subsystem 72 also directs the current image received from thecamera to the image display monitor 41. The vision subsystem 72 handlesadjustment of the lighting 95 in the inspection cavity as well.

The resolution and zoom capabilities of the camera must be selectedaccording to the size, density and appearance of the defects beinginspected. A full color camera having a resolution of at least 2000×2000pixels is preferred to discriminate features both by light intensity andwave length, allowing identification of markings on color codedcomponents. However, where defects are discernible in monochrome, andextremely fast inspection is required, a digital black and white cameramay be used. While an analog signal camera may be employed, a digitalcamera is preferred for both speed and accuracy.

Defects are discovered using digital image analysis techniqueswell-known in the art such as by testing the scanned images against atable of expected or otherwise acceptable values for certain features,such as space between terminal leads. Preferably, testing involves anumerical, rule based analysis of the images. The type and location ofany defect is then added to the database record for the current boardbeing inspected. A decision is also made regarding whether the detecteddefects are severe enough to require rework of the board.

The best mode vision subsystem uses the OMEGATEK 2000 brand digitalimage analyzing model developed by omegatek, Inc. of San Diego, Calif.which includes a full color digital camera having resolution of2000×2000 or 4 million pixels and a fixed or auto zoom lens. Thisparticular type of image analyzing model is based on a 200 MHz PENTIUMbrand microprocessor and uses proprietary image processing algorithms.

Using a camera having a resolution of at least 4 million pixels,features as small as approximately 12.5 microns (0.5 mils) have beenadequately discriminated. Using the OMEGATEK 2000, an averagelypopulated 3 inch×5 inch board such as standard PCI SVGA video adaptercard will take about 20 to 30 seconds to inspect thoroughly. Inspectionspeed can be further increased by adding more and faster processors toeither or both systems.

While similar systems can tolerate imprecision of the camera carriage of0.001 of an inch, the invention can accommodate imprecision of 0.02 ofan inch. Additionally, the carriage does not have to be as precise orfast as similar systems, thus a less expensive mechanism may beemployed. Additionally, it is not necessary to put down stabilizingsupports.

As a result of the high resolution to the vision subsystem, typicalinspections can be done using a single image from a single scannedcamera frame. However, if finer detail is required the camera can“zoom-in” and the inspection can be done using multiple images. Thedistance between the camera and the board typically ranges from 5 toinches, as limited only by the enclosure size with the average distanceat 7.2 inches. Although the usual height of components is 0.5 to 0.75inches, unusually tall components such as those 2 inches high, can beaccommodated, making reconfiguration of the vision subsystemunnecessary.

Being tolerant of much lower precision movement and placement translatesdirectly into a system that is lighter weight and less expensive. Beingmore portable, the station can easily be moved to different locations inthe assembly line, moved out of the line as a manual inspection orrework station, or off-site.

Referring now to FIG. 3, there is shown the automated embodiment anelectronic assembly video inspection station 56. The physical layout ofthe automated station is governed by its compatibility with standard SMTautomated assembly processes. The station comprises a housing 2 havingan inner inspection cavity 3 with a lower surface 4 upon which traversesa board handling and transport mechanism, including a conveyor 5 forcarrying a circuit board 6 to be inspected.

The conveyor 5 receives the board 6 through an aperture 7 in a side wall8, then positions it under the camera 20. After inspection, the board 6continues along the conveyor 5 toward another aperture 9 in an oppositewall 10 through which the board is ejected. The inspection cavity isaccessible for maintenance through a pair of access doors 11,12. Duringmanual operation individual boards may be loaded for inspection throughthese doors, or through a precision alignment drawer described below.

The video camera 20 is mounted to a movable, motorized carriage 21attached via a track 22 to the ceiling of the cavity 3. The carriageallows for directed width 23 and depth 24 movements of the camera in aplane substantially parallel to the plane of the board 6 beinginspected. This allows for the station to inspect boards of varyingsizes and complexities.

The carriage is preferably moved via servo motors such as a pair ofstepping motors powered with a pulsing signal generated by the carriagecontroller that induces incremental rotational movement of each motor.Each motor in turn drives the carriage along a pair of orthogonallyoriented precision linear slides. The first slide is mounted to theenclosure and the second to a portion of the carriage.

Below the inspection cavity 3 is a cabinet 25 housing the mastersubsystem computer 26 and an electronics bay 27 housing components ofthe vision subsystem and the various controllers and interfacesnecessary for signaling the components of the station, such as theconveyor and carriage controllers. Manual controls such as a powerswitch 30, conveyor overrides 31, conveyor pneumatic pressure controls32, carriage overrides 33, and an emergency shut-down switch 34, as wellas various status indicators extend from a front panel of the cabinet.Height adjustable legs 35, 36 allow for vertical positioning andorientation of the station housing 2. While the end of the legs mayterminate in foot pads 35 a, 36 a, locking wheels 35 b, 36 b arepreferable as shown in the manual inspection station embodiment of FIG.4.

Atop the station housing 2 are a pair of monitor displays 40, 41providing user interface screens to the analysis and control system, andimages recorded by the camera. An overall station status light tree 42is also provided.

The inspection station 56 is preferably adapted from an existing SMTdispensing station such as the Model No. A-618C and sold commerciallyfrom Asymtek, Inc., of Carlsbad, Calif. Using this relatively lowprecision system, the entire inspection station weighs under 250kilograms. This dispenser provides most of the major componentsdescribed above in an integrated package. A detailed description of thedispenser is available in the A-612/618C System Operations Manualavailable from Asymtek, which is incorporated herein by this reference.

In general the A-618C provides the housing and board handling mechanismsincluding a pneumatically driven conveyor and its controller andinterfaces, the carriage and its controller and interfaces, and thenecessary SMEMA controllers and interconnections. The dispenser isadapted by replacing its fluid injection equipment. There is no need toreplace the existing low precision carriage 21, since the highresolution vision subsystem can tolerate it. The height of the carriage21 may be adjusted to allow proper focal distance between the camera 20and the board 6. The main computer of the dispenser assumes the tasks ofthe master subsystem of the inspection station. Standard monitors arepreferably replaced with Super Video Graphics Array (“SVGA”) typemonitors for user interface 40 and monitoring camera image 41.

The A-618C provides for translation of the carriage over a range whichwill accommodate boards of approximately 46×46 centimeters (18×18inches).

Even in the most automated system, human intervention is often requiredto maintain the proper alignment of integrated mechanical components andto monitor the station's progress. Therefore, an operator can observethe image being viewed on the image monitor 41, and query the status ofthe inspection using the interface monitor 40 and data entry devices 82.If necessary, manual operation of the inspection station is availablethrough manipulation of. manual controls 30–34.

Board inspection is a multi-step process. First, the vision subsystem isconfigured according to the type of board to be inspected. Parametersrelating to the type, location, and orientation of each component to beinspected on the board are loaded into the vision subsystem via userdata entry. Preferably, the information is available via CAD (ComputerAided Design) files or other well known formats. Previous configurationsare stored on the master subsystem for quick retrieval from subsequentscans of the same board type.

Although a given run of an SMT process is typically configured for asingle type of board, it should be noted that the inspection system neednot be configured for only one type of board on any given run. So longas the system is able to identify each board being inspected, a largenumber of different typed boards may be successively inspected. This isof particular value in the optional rework inspection station 60 wheremany successively different boards types may require inspection. Indeed,the rework station may be serving a plurality of SMT lines.

Once the system has been configured for a particular type of board, theinspection process may begin. A circuit board 6 is positioned, face-up,on the conveyor 5 at a so-called “zero-position”. The conveyor 5 movesthe board 6 to a beginning position under the camera 20. Since thevision system is able to calculate its position from the visiblefeatures on the board, highly precise movement of the conveyor andcarriage is unnecessary. The inspection can take place with the board 6remaining on the conveyor 5, eliminating the need for any kind oflocking cradle and any board handling mechanisms required to place theboard on or remove board from the cradle, further reducing cost andweight, and increasing portability and speed of inspection. However,board locking cradles may be used.

The vision subsystem first identifies the board by reading a barcode orother visible board identifying cue on the board's surface such as a barcode. The vision subsystem capable of identifying and isolating the barcode and ascertaining its identification of the board. Hence, separatebar code reading equipment is not necessary. A record relating to theboard is retrieved by the master subsystem 26 for recording informationabout the board generated by the vision subsystem and any systemreconfiguration is automatically done by the master subsystem, withoutuser interface. If a record does not exist, the master subsystem willquery the operator for descriptive input as described above.

The vision system performs a scan of the board to obtain an image foranalysis. The scan may be administered first with a relatively coarseresolution followed by one or more subsequent scans in finer detail toresolve the status of questionable regions discovered during the coarsescan. Preferably, a single frame from the camera is used to increasethroughput.

After inspection is complete, the board is ejected for routing to thenext SMT station or to the rework station. When rework is necessary, allpertinent information for the rework is automatically available to therework system. At the rework station a user enters or scans the boardidentification via a bar code reader or other input device. Datapertaining to that particular board is then retrieved from the databaseand graphically displayed on the rework monitor. The graphical displaypreferably comprises a stock image of the type of board being reworked.A stock image is used to reduce the amount of data to be sent to therework station. Overlaid on the image, at the instruction of the reworkoperator, are visual icons, analogous to red dots on a map showing thelocation and type of defects for that board. A CAD overlay, and anyother pertinent information that has been entered in the database andmay also be overlaid or otherwise displayed. Alternately, a simpletextual printout of defects can be made available to the rework station.

Referring now to FIG. 4, when operated as a manual inspection stationfor “batch processing”, a drawer 43, is available to manually insert aboard for inspection. The drawer 43 is located in the front of thestation 56 a and slides out toward the user. In this embodiment, thestation 56 a is not part of a SMEMA line; therefore, the light tree 42is unneeded. In addition, both the camera and lights are mounted on thecarriage 22 a behind a box-like screen 45. The monitors 40 a, 41 a,master computer 26 a, and the various switches and indicators have beenrelocated. A keyboard is mounted within a second drawer 46.

Referring now to FIGS. 5–8, the drawer 43 is comprised of a flat surface48 with a plurality of die holes 47 drilled through at regular intervalsboth horizontally and vertically. The surface on which the board isplaced is approximately parallel to the plane in which the cameratravels. A switch 49 with its data line 50 is located on the holedsurface 48, which detects the presence of a board 6.

Able to be affixed to the holed surface 48 are two types of attachmentswhich secure the board in place. The first type are simple elongatedpiers 80, 81 through which finger tightened screws 83 engaged the holedsurface. Typically, for rectangular circuit boards, these piers would beplaced at right angles to one another to engage adjacent sides of theboard.

The second type of attachment 84,85 are adjustable brackets which engagethe other sides of the board. Each bracket is attached to the holedsurface by a pair of finger tightened screws 86 passing through anoblong slot 87. When the screws are partially tightened, the bracket canslide back and forth in a line along the holed surface.

Referring now to FIG. 6, each oblong pier 80 is made from strong, rigidmaterial such as aluminum. The top of each pier has a horizontal planarupper surface 100 extending from the top inner edge 101 outward to avertical planar wall 102 running longitudinally along a median portionof the top of the pier. The height 103 of the wall is roughlycommensurate with the thickness of the circuit board 6 in order toadequately secure it. More outwardly located are one or more clampingknobs 104, 105 rotatively mounted to the top of the pier. Eachadjustable bracket 84, 85 has a similar board securing upper surface andone clamping knob.

Each knob is generally cylindrical shaped having a vertically planarcutaway 106 placed a radial distance from the axis of rotation 107 nogreater than the distance from the axis to the wall 102. This allows forextraction of the board when the cutaway is brought into alignment withthe wall as shown in FIG. 7. When out of alignment as shown in FIG. 8, aportion 108 of the bottom surface of the knob extends over and contactsa portion of the top surface of the seated board 6.

Means for maintaining the position of the knobs may be used such asbiasing the knob downward through use of a spring 109. Other well-knownmeans may also be used.

Although the instant invention is directed toward the inspection ofelectronic assemblies such as printed circuit boards having surfacemounted components, it is appreciated that the invention applies toother manufactured items having defects which are detectable throughvisual inspection.

Although in the preferred embodiment describes the camera being movablein a plane, it is appreciated that more complex motion involvingvertical movement, and pitch, yaw, and roll movements may be added tothe system depending on the type of article being inspected.

While the preferred embodiments of the invention have been described,modifications can be made and other embodiments may be devised withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

1. A method for visually inspecting electronic assemblies of a varietyof components, said method comprises: positioning one of said assembliescomprising a printed circuit board mounting a plurality ofsurface-mounted components of different types, orientations and terminallead spacings, within the view of a digital camera; obtaining via saidcamera, an image of said assembly; scanning said image to obtain arecord of observed features including type, location and orientation ofa variety of surface-mounted components, and terminal lead spacing data;maintaining a database of a plurality of visually perceptible parametersof said features; and numerically analyzing each of said observedfeatures under a plurality of said visually perceptible parameters tolocate defects therein including component placement, lead spacing andorientation defects.
 2. The method of claim 1, wherein said analyzingcomprises using a numerical rule-based color image analysis algorithm.3. The method of claim 1, wherein said numerically analyzing comprises:comparing said image against a record of acceptable values; andidentifying areas of said image which do not fall within said values. 4.The method of claim 3, wherein said maintaining comprises: enteringassembly layouts including component type locations and orientationsinto said database.
 5. The method of claim 4, wherein said enteringcomprises interpreting a computer-aided design file describing at leastone of said assemblies.
 6. The method of claim 5, which furthercomprises: identifying each of said assemblies via a visual cue on asurface of said board read by said camera; and categorizing each of saidassemblies within one of a plurality of assembly types.
 7. The method ofclaim 6, wherein said step of positioning comprises: journaling each ofsaid assemblies along a conveyer; and securing each of said assemblieswithin view of said camera, wherein said securing has an acceptabletolerance coarser than two hundredth of an inch.
 8. The method of claim1, wherein said obtaining comprises: scanning a first course resolutionimage of said assembly; identifying a potentially defective region ofsaid assembly; and scanning a first fine resolution image of saidregion.
 9. The method of claim 1, which further comprises catalogingsaid list of defects.
 10. The method of claim 1, wherein said step ofgenerating comprises: displaying a template representing said assembly;displaying a distinct static visual icon for each of said defects at alocation on said template corresponding to the location of each of saiddefects on each of said assemblies.
 11. The method of claim 10, whereinsaid visual icon comprises indication of a type of defect.
 12. Themethod of claim 1, wherein said camera is mounted with an enclosuresupported by wheels.
 13. The method of claim 1, wherein said obtainingcomprises positioning said camera a distance of at least 12.7centimeters above said board.